This invention relates to photoflash lamps and, more particularly, to flashlamps of the type filled with combustible material ignited in a combustion-supporting atmosphere by an electrically conductive filament.
A typical photoflash lamp comprises an hermetically sealed glass envelope containing a quantity of combustible fill material, such as shredded zirconium or hafnium foil, and a combustion-supporting gas, such as oxygen, at a pressure above one atmosphere. In electrically ignitable photoflash lamps, the ignition means typically comprises a pair of lead-in wires sealed through and extending inside one end of the tubular glass envelope. A tungsten filament is mounted across the inner ends of the two lead-in wires with the ends of the wires at their junctions with the filament being coated with a primer material.
Operation of the photoflash lamp is initiated by an electrical current pulse supplied across the external ends of the two lead-in wires. The current flow through the ignition filament causes it to be heated to an elevated temperature. This ohmic heating of the tungsten filament wire conductively heats the beads of primer material contacting the filament wire. When the beads of primer material reach their ignition temperature, they deflagrate and expel burning metal powder throughout the lamp volume. This, in turn, ignites the shredded metallic combustible material, which upon burning generates the actinic output of the lamp.
Throughout the history of photoflash lamps there has been a continuous trend of miniaturization of the lamp envelope. If we look at several photoflash lamps in the order of their development, such as the P-25, M-3, AG-1, FC-1, and HP-1, their internal volumes are 31.0, 7.5, 1.2, 0.62, and 0.78 cc, respectively. With these same lamps, as the volume decreased, the internal O.sub.2 pressure increased. Again, in the order of lamp type mentioned, the internal pressures are 53, 95, 360, 500, and 675 cm. Hg., respectively. With the evolution of these lamp types, the primer formulation used was essentially unchanged. This material has basically followed the original composition as disclosed in U.S. Pat. No. 2,280,598, namely, a mixture of zirconium powder (2.5 microns), magnesium powder (325 mesh), potassium perchlorate (reagent grade), and a nitrocellulose binder. This fulminating material is applied to the lead-in wires in the form of a paste. Once dried, this material forms a sensitive coating for ignition of the flash lamp. A typical formulation now used is 83.0 % zirconium, 9.8% potassium perchlorate, 4.4% magnesium, and 2.8% nitrocellulose binder resin.
The mechanism of primer ignition is based on the fact that a sensitive mixture of fuel (magnesium and zirconium) is employed with an oxidizer (potassium perchlorate). With the input of electrical energy and subsequent heating of the filament, the fuel begins to ignite. The potassium perchlorate provides oxygen for supporting the combustion of the fuel. Once ignition begins with the release of oxygen from the potassium perchlorate, the reaction commences quickly and rapidly. Burning particles of fuel are distributed throughout the lamp envelope, causing ignition of the zirconium fill. Depending upon the lamp type, peak light intensity occurs anywhere from 12 to 20 milliseconds.
Primer compositions containing reactive fuels, such as zirconium and magnesium powder, together with highly active oxidizers, such as potassium perchlorate, are hazardous materials to prepare and handle in manufacture. When dried such mixtures are susceptible to ignition by friction, heat, or electrostatic discharges. U.S. Pat. No. 3,893,798 describes a primer having a lower ratio of oxidizer to fuel so as to reduce the hazards of handling primer solids in the manufacturing process.